Claw pole motor

ABSTRACT

An electronically commutated claw pole motor has an external rotor ( 42 ) with a shaft ( 40 ) and a rotor magnet ( 62 ). The claw pole motor contains a stator ( 64 ) which has a first soft ferromagnetic stator pole piece ( 74 ) located on its side facing away from the external rotor ( 42 ). The stator pole piece is provided with first claw poles ( 82, 84 ) projecting toward the external rotor ( 42 ). The stator has a second soft ferromagnetic stator pole piece ( 76 ) located on its side facing the external rotor ( 42 ). The second soft ferromagnetic pole piece is provided with second claw poles ( 88, 90 ) extending from the external rotor ( 42 ) and having a larger axial extension (h 2 ) than the first claw poles ( 82, 84 ) and projecting into gaps ( 92 ) between the first claw poles ( 82, 84 ). An axial bearing ( 44, 46 ) supports the end ( 44 ) of the shaft ( 40 ) which faces away from the external rotor ( 42 ). In order to generate a magnetic force (F) acting on this bearing, the rotor magnet is displaced, relative to the stator ( 64 ), in a direction away from the axial bearing ( 40, 44 ).

This application is a national phase application under § 371 ofPCT/DE98/02001 filed Jul. 17, 1998.

FIELD OF THE INVENTION

The invention relates to a claw pole motor, and in particular to anelectronically commutated claw pole motor.

BACKGROUND

Motors of this kind are often extremely small, and can have, forexample, an outside diameter of less than 2 cm. They are preferably usedto drive small fans for direct cooling of the processor in computers,and since only very limited space is available on a computer circuitboard, a low overall height for such motors is desirable; theiroperation must nevertheless be absolutely reliable.

A motor known from EP 766 370 A2 and corresponding U.S. Pat. No.5,831,359, JESKE, has a relatively large overall height, since in it,the interaction of the permanent magnet of the external rotor and theclaw pole arrangement of the interior stator must generate on the rotora magnetic pull which ensures that the shaft is pressed with sufficientforce onto the axial bearing. For this purpose, in this motor the rotorand stator must be highly offset in the axial direction.

It is therefore an object of the invention to make available a new clawpole motor, in particular a subminiature motor, that is suitable, forexample, as a fan drive system for cooling microprocessors.

SUMMARY OF THE INVENTION

According to the invention, this object is achieved by asymmetricalshaping of the claw poles, to thereby displace the magnetic symmetryplanes of the stator and rotor. It thereby becomes possible either toincrease the axial magnetic pull acting on the rotor, or to reduce theoverall height of the motor, or also to take a combination of thesemeasures.

Further details and advantageous developments of the invention areevident from the exemplary embodiments—which are not to be understood inany way as a limitation of the invention—that are described below.

BRIEF FIGURE DESCRIPTION

FIG. 1 shows a longitudinal section through a fan having anelectronically commutated subminiature claw pole motor according to theinvention, at greatly enlarged scale; in reality a motor of this kindcan have, for example, a diameter of approximately 2 cm;

FIG. 2 shows a developed view of the claw poles of the claw polearrangement used in the motor of FIG. 1;

FIG. 3 shows a plan view of upper pole piece 74 of FIGS. 1 and 2, viewedin the direction of arrow III of FIG. 4;

FIG. 4 shows a side view of upper pole piece 74;

FIG. 5 shows a schematic depiction to explain the generation of amagnetic force;

FIG. 6 shows a schematic depiction of a detail;

FIG. 7 shows a variant of the stator configuration of FIG. 2;

FIG. 8 shows an electrical circuit for commutating the motor accordingto the Figures listed above and the motor according to FIGS. 12 and 13;

FIG. 9 shows a side view of the lower pole piece of a further embodimentof the invention;

FIG. 10 shows a plan view of the pole piece of FIG. 9, viewed in thedirection of arrow X of FIG. 9;

FIG. 11 shows a detail of the pole piece of FIGS. 9 and 10, viewed inthe direction of arrow XI of FIG. 10 and in a developed view;

FIG. 12 shows a depiction of a claw pole motor having a lower pole pieceaccording to FIGS. 9 through 11; and

FIG. 13 shows a detail XIII of FIG. 12 in a greatly enlarged depiction.

DETAILED DESCRIPTION

The invention is preferably utilized in very small motors, and istherefore described below using the example of a fan for cooling amicroprocessor, i.e. a so-called processor fan. Since such fans areextremely small, the drawings must be very greatly enlarged. For bettercomprehension, a scale bar 1 cm in length is shown as an example on theright in FIG. 1 in order to illustrate the size relationships. Adepiction at 1:1 scale obviously would not have been possible.

The reference number 10 designates a microprocessor (or othersemiconductor) that must be positively cooled because it generates agreat deal of heat. Mounted on it in thermally conductive fashion, inthe usual manner, is a heat sink 12 made of metal. This has cooling fins14 and (at the left) a threaded hole 16 onto which the housing 20 of aprocessor fan 22 can be mounted by way of a screw (indicated onlyschematically in FIG. 1 with a line 18). This housing 20 has anapproximately cylindrical air passage 24 in the center of which the baseportion 28 of an electronically commutated claw pole motor 29 is mountedvia radial spokes 26, only one of which is visible.

A soft ferromagnetic bearing support tube 32 is pressed into adepression 30 in the center of base part 28. A circuit board 34 is alsomounted on base part 28. As depicted, a Hall IC 35 (also depicted inFIG. 8) is mounted in a recess 37 of circuit board 34.

Pressed into bearing support tube 32 is a plain bearing (sinteredbearing) 36, which supports shaft 40 of an external rotor 42 whoserotation axis is labeled 43. At its upper, free end 44, shaft 40 restsagainst a bearing surface 46 of base part 28 and is thereby axiallysupported. For this purpose, a force F that is magnetically generatedacts on it upward. The generation of this magnetic force F is explainedbelow in more detail. It is important that this force be of sufficientmagnitude that, even in the position depicted, in which it hangsdownward, external rotor 42 can be reliably retained on the stator.

Mounted on the upper end region of shaft 40, as an axial retainer, is awasher 41 which slings oil outward (from sintered bearing 36) thatmigrates upward along shaft 40, and from there migrates back to sinteredbearing 36.

The lower end of shaft 40 is mounted in rotor base 50 of a rotor cup 52(manufactured from plastic) with which multiple fan blades 54 areintegrally configured, two thereof being visible. As rotor 42 rotates,blades 54 transport air in the direction of arrows 56, i.e. from bottomto top; in other words, the hot air between cooling fins 14 is drawn offupward. This results in a downwardly acting force on rotor 42 that inthis case, for example, has a magnitude of 0.05 N (at a rotation speedof 3900 rpm). This force acts against force F. The weight of rotor 42also results in a force of, for example, 0.1 N that acts against forceF, i.e. the latter must be greater than 0.05+0.1=0.15 N. In practice,therefore, the magnetically generated force F must in this example havea magnitude of at least 0.2 N in order to ensure reliable axial supportof rotor 42 in all positions of fan 22.

Mounted in the interior of rotor cup 52 by plastic injection molding isa soft ferromagnetic armature piece 60; mounted in that is a radiallymagnetized annular magnet 62 (rotor magnet) that can be magnetized inthe circumferential direction in four-pole and (preferably) trapezoidalfashion. Its upper end is located approximately opposite Hall IC 35, sothat rotor magnet 62 controls Hall IC 35 with its leakage field. Asdepicted, Hall IC 35 is radially offset slightly inward with respect tothe upper end of magnet 62. The advantages thereby obtained areexplained below with reference to FIG. 12.

Mounted on the outer side of bearing support tube 32 is an interiorstator 64, details of which are depicted in the subsequent Figures.Located in its interior is a winding former 66, on which is wound (asdepicted) an annular coil 68 that is therefore arranged concentricallywith shaft 40. Interior stator 64 is mounted on circuit board 34 by wayof rivets 70.

In the case of the present motor 29, magnetic force F is generated usingan asymmetrical configuration of the magnetic circuit of stator 64. Thelatter contains, as already described, winding former 66 with annularcoil 68, and this winding former 66 is located, in the case of a clawpole motor, between two stator pole pieces, i.e. an upper pole piece 74and a lower pole piece 76. Upper pole piece 74 has an annular innersegment or collar 78 that presses onto bearing support tube 32 and isthereby mounted on it. Lower pole piece 76 also has an annular innersegment or collar 80 that is also pressed onto bearing support tube 32.Winding former 66 with its coil 68 is thus held between pole pieces 74,76.

Upper pole piece 74 rests directly on circuit board 34 and, as shown inFIG. 2, has two claw poles 82, 84 of identical shape that projectaxially downward from above into corresponding gaps 86 that areconfigured between the upward-facing claw poles 88, 90 of lower polepiece 76. Claw poles 88, 90, conversely, project into gaps 92 betweenclaw poles 82 and 84.

The shape of upper pole piece 74 is best appreciated from FIGS. 3 and 4,in which the rotation direction of rotor 42 is labeled R. It is evidentthat claw poles 82, 84, proceeding from their leading edges 96 and 98,each become wider in the rotation direction in an angular region α, andfor that purpose have an oblique edge 100 and 102. In the adjacentrotation angle region β, their width h1 is constant (in terms of theaxial direction). Typical values are α=60° el., β=100° el.

In a usual claw pole motor such as shown, for example, in EP 766 370 A2,the claw poles of the upper pole piece and lower pole piece areidentical, i.e. in the known motors, the interior stator has asymmetrical structure.

In contrast thereto, in the case of the present invention, pole pieces74 and 76 are of different configuration (cf. FIG. 2) in order to yieldan asymmetrical configuration of the stator. This asymmetricalconfiguration meets the additional condition of making possible areluctance torque shape that is necessary for operation of the motor.(The reluctance torque occurs upon rotation as a result of coactionbetween the permanently magnetized rotor 62 and the soft iron masses ofthe claw poles that are located opposite rotor 62.)

For better comprehension, FIG. 2 depicts rotor magnet 62 (in sectiononly, and sketched) and axial bearing 44, 46. Rotor magnet 62 has amagnetic center plane 106 that runs perpendicular to rotation axis 43.This center plane 106 would, if a homogeneous magnet were cut apartthere, divide it into two substantially identical rings. Interior stator64 has a geometrical center plane 108, i.e. if stator 64 has a totalheight H, its geometrical center plane 108 is at a distance H/2 fromboth axial end faces 110, 112 of stator 64.

The two claw poles 88, 90, of which only one is depicted completely inFIG. 2, have the same shape, so that a description and depiction of clawpole 90 will suffice. At its leading edge 116, the latter has a cutout118 of length α adjoining which is a portion of claw pole 90 with lengthβ; no cutout is present in the latter part.

A characteristic of cutout 118 (or 118′ in claw pole 88) is that ittapers, i.e. becomes narrower, in rotation direction R. This isaccomplished preferably by way of an oblique profile on its upperperiphery 120 or 120′, while lower periphery 122 or 122′ runsperpendicular to rotation axis 43 and is located as far down aspossible. Conversely, however, periphery 122 or 122′ could also extendobliquely, and periphery 120 could extend perpendicular to rotation axis43. In this case, however, force F would be smaller.

Oblique periphery 120 or 120′ runs substantially parallel to edge 100 ofclaw pole 82, in terms of the developed view of FIG. 2.

The result of this configuration of claw poles 88, 90 is that part 126of these claw poles, depicted in gray in FIG. 2 for claw pole 90, isshifted one “floor” upward, so to speak, as compared to a pole pieceshown in FIGS. 3 and 4.

In addition, one proceeds according to the invention preferably in sucha manner that axial extension h1 of upper claw poles 82, 84 is smallerthan axial extension h2 of lower claw poles 88, 90, i.e. axial gap s1between lower pole piece 76 and lower end 82′, 84′ of upper claw poles82, 84 is larger than axial gap s2 between upper pole piece 74 and theupper ends of lower claw poles 88, 90. (An embodiment in which h1=h2would also be possible, but force F would then be smaller.)

The result thereof is that a force f1, which attempts to bring magneticcenter plane 106 of rotor 62 into alignment with magnetic symmetry plane130 of stator 64, acts in an upward direction on rotor magnet 62, whichis located in the position shown in FIG. 2. This magnetic symmetry plane130 of stator 64 lies a distance d higher than its geometrical centerplane 108. This is therefore a consequence of the “height offset” ofsegments 126, and of the different gaps s1 and s2.

There is also a second effect: As already described, rotor magnet 62controls, with its axial leakage field, Hall IC 35, that is arranged ina recess 37 of circuit board 34. This Hall IC 35 is also depicted inFIG. 2 for illustrative purposes. In order to allow this control action,only a small axial air gap 134, for example of 0.3 mm, is presentbetween Hall IC 35 and the upper end of rotor magnet 62. As a result,rotor magnet 62, which preferably has the same height H as stator 64, isoffset downward with respect to the latter by a magnitude equal to saidair gap 134, i.e. for example by 0.3 mm; this results in an additionalupward-acting force f2on rotor magnet 62, since at both axial ends ofrotor magnet 62 the magnetic fields are distorted by this offset, andthis distortion results in force f2. This applies in particular in thecase of the practically rectangular magnetic flux density distributionexisting here in the axial direction on rotor magnet 62.

Forces f1 and f2add up to yield the magnetically generated force F=f1+f2, which reliably pulls rotor 42 upward as shown in FIG. 1, andreliably holds free end 44 of shaft 40 in contact against bearingsurface 46.

FIG. 5 explains the generation of force f2. Magnetic field 140 betweenrotor magnet 62 and stator 64 (indicated only schematically) has asubstantially homogeneous profile in air gap 142.

In the region of upper edge 144 and lower edge 146 of rotor magnet 62,however, this field is distorted in the manner depicted schematically at148 and 150 in FIG. 5. This is the case assuming that length HR of rotor62 is substantially identical to height HS of stator 64. If magneticfield lines 140 are imagined to be rubber filaments, these field linesare, so to speak, stretched at both edges 144 and 146 and thereforeattempt to pull rotor magnet 62 upward, so that force f2is generated inthis position of rotor 62.

Force f1is generated by the fact that the magnetic flux density in theupper region of stator 64 is somewhat higher—since more iron is presentthere because of the claw poles—than in the lower region; this is notdepicted in FIG. 5.

FIG. 6 shows, in a partial depiction, a further detail to explain themaximum gap width s1 (cf. FIG. 2). Experiments have shown that upperclaw poles 82, 84 should be of a length such that they cover coil 68, asdepicted in FIG. 6.

If h1 is made smaller than in FIG. 6, so that coil 68 is not covered,the result of the increased size of gap s1 is to generate leakage fluxesfrom coil 68 and to generate axial vibration forces on rotor 62, thuscausing additional motor noise.

With a symmetrical configuration of stator 64, such axial forces wouldnot occur even if the claw poles of both pole pieces were greatlyshortened in the same fashion, since the leakage fluxes then cancel oneanother out. In the case of the asymmetrical design as shown in FIGS. 2,7, or 9 through 12, however, with different gaps s1 and s2, the rulethat should be followed is that h1 must be of sufficient size that upperclaw poles 82, 84 cover coil 68, if it is desirable to eliminate theaforesaid motor noise.

FIG. 7 shows a variant of the stator configuration of FIG. 2. The statorshown in FIG. 7 is labeled 64′. Upper pole piece 74 with its claw poles82 and 84 corresponds to the construction already described in detailwith reference to FIGS. 2 through 4.

Lower pole piece 76′ has in its claw poles 88′, 90′ cutouts 118″ and118″′, and the latter contain reinforcing struts 156, 156′ that providemechanical reinforcement of claw poles 88′, 90′. It must be kept in mindin this context that these claw poles are very small and thin, andtherefore could very easily be deformed under mechanical stress. Itwould be ideal to manufacture these reinforcing struts 156, 156′ fromnonmagnetic material, e.g. from brass or plastic. These reinforcingstruts should preferably extend at an angle to the axial direction inorder to prevent disruptive magnetically generated torques.

This design thus results in the creation, in lower claw poles 88′, 90′,of closed openings 158, 158′ that magnetically constitute a part ofrecesses 118″ and 118″′, as is evident from a comparison of FIGS. 2 and7.

In FIG. 7, struts 156, 156′ run parallel to rotation axis 43, butpreferably could also run at an angle to it. Instead of only one strut,two or three could also be provided, which can then be of thinnerconfiguration and thus can more easily be saturated magnetically, whichis advantageous here. Gap s2′ is depicted here as being larger than gaps2 of FIG. 2. This gap should be as small as possible; it can also, ifapplicable, extend radially.

FIG. 8 shows a circuit for operating a motor 29 according to the presentinvention whose rotor magnet 62 is indicated schematically. Coil 68 hasa two-wire winding and therefore has two strands or phases 68′ and 68″which are depicted in FIG. 8. To supply power to Hall IC 35, the latteris connected at one terminal to a positive line 162 and via a resistor164 to a negative line 166. Its output 168 is connected via a pullupresistor 170 to positive line 162, via a resistor 172 to the base of annpn Darlington transistor 174, and via a resistor 176 to the base of annpn transistor 178 that serves as a phase reversal transistor. Theemitter of transistor 178 is connected to negative line 166; itscollector is connected via a resistor 180 to positive line 162, and viaa resistor 182 to the base of an npn Darlington transistor 184.

The emitters of transistors 174, 184 are connected to one anotherand—via a common emitter resistor 185—to negative line 166. Thecollector of transistor 174 is connected via winding strand 68′ topositive line 162. The collector of transistor 184 is similarlyconnected via winding strand 68″ to positive line 162. Located betweenthe collector and base of each of transistors 174, 184 is a capacitor186, 188 (e.g. 3.3 nF) whose function is to slow down the switchingoperations and thereby prevent high-frequency interference duringcommutation.

When the axial leakage field of rotor magnet 62 causes the signal atoutput 168 of Hall IC 35 to become high as rotor 42 rotates, transistors174 and 178 become conductive. Strand 68′ thereby receives current, andtransistor 184 becomes nonconductive because its base is connected viatransistor 178 to negative line 166.

Conversely, when the signal at output 168 of Hall IC 35 becomes low as aresult of the magnetic field of rotor magnet 62, transistors 174 and 178become nonconductive. Transistor 184 then receives via resistor 180 abase current that makes it conductive, so that in this case strand 68″receives current and phase 68′ is currentless.

The potential at output 168 of Hall IC 35 is controlled by the fact thateither a north pole or a south pole of rotor magnet 62 is locatedopposite said Hall IC, i.e. the currents in phases 68′, 68″ arecontrolled by the rotational position of rotor 62.

A diode 192 prevents motor 29 from damage due to mispolarization.Capacitors 194, 196 serve to filter out interference pulses to ensurethat the motor runs quietly, and also prevent interference voltages fromescaping from motor 29.

Since it is possible, in the above-described motor 29, for rotor 62 tohave a small axial length HR but nevertheless for a sufficient axialforce F to be generated, the overall result is a considerably reducedoverall height for the above- described processor fan 22 (or any otherdevice), along with reliable operation.

In the variant according to FIGS. 9 through 13, upper pole piece 74 hasthe same shape that has been described in detail with reference to FIGS.3 and 4.

Lower pole piece 206 is similar in configuration to lower pole piece 76′of FIG. 7. According to FIGS. 9 and 10 it has a flat segment 208 with aninner collar 210, and it has two claw poles 212, 212′ that aresymmetrical in shape; only claw pole 212 will therefore be described.The parts of claw pole 212′ are labeled with the same referencecharacters as the parts of claw pole 212, but with the addition of anapostrophe, i.e., for example, 212′ rather than 212.

In an angular region α′ claw pole 212 has an iron volume that increasesin rotation direction R, and in an adjacent rotation angle region β′ ithas a constant iron volume. For this purpose, this claw pole has onleading rim 214 an approximately semicircular cutout 216 that, however,does not extend to upper edge 218 of this claw pole 212, but rather endsbelow it, as depicted.

Adjoining this cutout 216 is a strut 220 whose function is to impart toclaw pole 212 the necessary mechanical strength; and strut 220 isadjoined by an approximately circular cutout 222 whose nature is suchthat the iron volume of claw pole 212, as already explained, generallyincreases in rotation direction R in rotation angle region α′, or,stated conversely, that recesses 216, 222 have a decreasing tendency inthat rotation angle region.

In the adjacent rotation angle region β′ (FIG. 10), claw pole 212 hasits full axial extension, i.e. its maximum iron volume, and it ends attrailing edge 224.

Since the motor in FIGS. 12 and 13 is identical in mechanicalconfiguration, in all essential aspects, to FIG. 1, the referencecharacters used for identical parts are the same as in FIG. 1, and theseparts are not described again.

Here again, Hall generator 35 is offset radially inward in circuit board34, so that its center lies approximately above radial inner edge 62′ ofrotor magnet 62. It has been found that Hall generator 35 can be morereliably controlled by the magnetic field of rotor magnet 62 with anarrangement of this kind. This is because Hall generator 35 iscontrolled by the leakage field of the radially magnetized magnet 62,and the leakage field attains its greatest magnetic flux density in theposition depicted.

In the arrangement according to FIGS. 9 through 13 as well, a magneticforce F is created that presses shaft 40 against bearing surface 46. Thegeneration of this force F has already been described in detail inconjunction with FIGS. 2, 5, and 7, to which reference is thereforemade. What emerges as essential in the embodiment according to FIGS. 9through 13 is that the shape of strut 220 achieves a good balancebetween the magnetic requirements of the motor (optimal shape for thereluctance torque that is generated) and the magnetic strengthrequirements of said strut and thus of claw poles 212, 212′.

Angles α′ and β′ in FIG. 10 are substantially identical to angles α′ andβ in FIGS. 2 and 3.

Many variants and modifications are of course possible within thecontext of the present invention.

What is claimed is:
 1. In a claw pole motor, comprising an interiorstator having two soft ferromagnetic stator pole pieces provided withclaw poles formed by ferromagnetic masses and separated by gaps, eachclaw pole projecting into an associated gap between two claw poles of astator pole piece located opposite, said stator having a geometricalcenter plane; further comprising an external rotor rotatable on arotation axis in a circumferential direction and in which is arranged arotor magnet having, with respect to its axial extension, a magneticcenter plane extending transverse to the rotation axis, and having anaxial bearing associated therewith for defining its axial positionrelative to the stator; the external rotor being acted upon, in adirection toward said axial bearing, by a magnetic force (F) effectivebetween the interior stator and the rotor magnet; the ferromagneticmasses forming said claw poles defining a magnetic symmetry plane insuch a way that the magnetic center plane of the rotor magnet, if saidrotor were freely displaceable in an axial direction, would align itselfwith said magnetic symmetry plane; said magnetic symmetry plane of theinterior stator being furthermore offset, with respect to a geometricalcenter plane thereof, toward the axial bearing, by asymmetrical shapingof said claw poles, said magnetic center plane of the rotor magnet beingaxially offset, relative to said magnetic symmetry plane of the interiorstator, in a direction away from the axial bearing.
 2. The claw polemotor as defined in claim 1, wherein the claw poles have, at leastpartly and at least in local regions, edges running obliquely relativeto the circumferential direction, so that, in these local regions, theirmagnetically effective width (h1, h2) increases in the rotationdirection (R) of the external rotor.
 3. The claw pole motor as definedin claim 2, wherein obliquely extending edges are provided in the samemanner on the side of the claw poles of said two stator pole pieces, andsaid obliquely extending edges are provided on the side of therespective claw pole facing away from the axial bearing.
 4. The clawpole motor according to claim 1, wherein in at least a portion of theclaw Poles projecting toward the axial bearing, at least one cutout isprovided, said cutout extending in the rotation direction (R) from aleading side edge of the respective claw pole into said claw pole with amagnetically effective width decreasing in the rotation direction (R).5. The claw pole motor as defined in claim 4, further comprising atleast one stiffening strut subdividing the cutout.
 6. The claw polemotor as defined in claim 5, wherein the at least one stiffening strutextends substantially in the axial direction.
 7. The claw pole motor asdefined in claim 1, further comprising a rotor base, a rotary shaft (40)fixed to the rotor base, and wherein said rotary shaft is held incontact against an axial bearing of the motor by a magnetic force (F)effective between the interior stator and said rotor magnet.
 8. The clawpole motor as defined in claim 1, wherein an axial extension of the clawpoles of the stator pole piece located closer to the axial bearing is atleast partially smaller than an axial extension of the claw poles of theother stator pole piece.
 9. In a claw pole motor; having an externalrotor in the form of a rotor cup, on whose base a shaft is mounted, andin which a rotor magnet is arranged; having a stator comprising, on itsside facing away from the rotor base, a first soft ferromagnetic statorpole piece provided with first claw poles projecting toward the rotorbase, and that comprising, on its side facing toward the rotor base, asecond soft ferromagnetic stator pole piece provided with second clawpoles projecting away from the rotor base and having a greater axialextension (h2) than the first claw poles and projecting into gapsprovided between the first claw poles; having an axial bearingsupporting the end of the shaft facing away from the rotor base; andsaid rotor magnet being axially offset, relative to the stator, in adirection away from said axial bearing in order to generate a magneticforce (F) acting on the axial bearing.
 10. The claw pole motor asdefined in claim 9, wherein the first stator pole piece is arranged on acircuit board on which a galvanomagnetic sensor is arranged in theregion of an end face of the rotor magnet provided on the rotor cup. 11.The claw pole motor as defined in claim 10, wherein said galvanomagneticsensor is arranged in a recess of the circuit board.
 12. The claw polemotor as defined in claim 11, wherein the galvanomagnetic sensor isradially offset, toward the rotation axis of the motor, with respect toa position directly opposite the end face of the rotor magnet.
 13. Theclaw pole motor as defined in claim 9, wherein the end face of the rotormagnet facing toward the galvanomagnetic sensor is separated therefromby an axial air gap constituting at least a part of the axial offsetbetween stator and rotor.
 14. The claw pole according to claim 13,wherein said rotor magnet has an axial length and said stator has anaxial length substantially identical with the axial length of said rotormagnet.
 15. The claw pole motor as defined in claim 9, wherein the axialbearing is configured as a plain bearing between a free end of the shaftand a stator-mounted part.